When a fuel cell system is operated for a long period of time, an electrolyte of a fuel cell stack degrades, an electrolyte membrane cracks, and differential pressure resistant properties lower, thereby causing cross leak sometimes. To solve this problem, heretofore, a system for detecting the occurrence of the cross leak has been developed. For example, Japanese Patent Application Laid-Open No. 2006-120375 discloses a fuel cell system in which when the concentration of hydrogen in air discharged from a fuel cell stack is not less than a predetermined value, it is judged that cross leak has occurred, to urgently stop a fuel cell (Patent Literature 1).
Moreover, another system has been developed in which during light loading or during idling, the power generation voltage of the fuel cell might rise to promote the degradation of the fuel cell, and hence the rise of the power generation voltage of a fuel cell is suppressed, thereby suppressing the degradation of the fuel cell (hereinafter processing to prevent the voltage of the fuel cell from rising to a constant value or more will be referred to as the “high-potential-avoiding processing”). For example, Japanese Patent Application Laid-Open No. 2007-109569 discloses still another fuel cell system in which as means for suppressing the degradation of a fuel cell by the high-potential-avoiding processing, an air compressor is operated so that a cell voltage is not more than a beforehand set predetermined upper limit voltage, and is controlled to intermittently supply air to a fuel cell stack (Patent Literature 2).
Furthermore, for example, Japanese Patent Application Laid-Open No. 2007-103023 discloses a further fuel cell system in which as a measure for preventing cross leak from occurring at the stop of the system, a technology is used to dispose of hydrogen crossing over an anode and accumulated in a cathode when the system is left to stand after the stop. At the start of a fuel cell, an oxidant gas is supplied under pressure until a predetermined pressure is obtained, and the gas is sealed between a shut valve and an air pressure regulation valve, whereby hydrogen present in a cathode path is burnt and treated on a catalyst in the cathode (Patent Literature 3).
In addition, there is another technology in which when the electrolyte membrane degrades owing to the adsorption of oxygen, the output voltage of the fuel cell is lowered while suppressing the supply of air, to operate the fuel cell in a reduction region, thereby activating a catalyst layer (this processing will hereinafter be referred to as the “catalyst activation processing”). As a technology concerned with such catalyst activation processing of the fuel cell, for example, Japanese Patent Application Laid-Open No. 2003-115318 discloses a technology in which a cell voltage is set to 0.6 V or less, and a large current is allowed to flow to cause a reducing reaction in oxygen, thereby activating a platinum catalyst layer (Patent Literature 4).
Moreover, if the amount of water included in cells constituting the fuel cell is not controlled in an appropriate range, the cells excessively dry, or a wet degree thereof is excessively high, thereby accelerating the degradation of the cells. To solve the problem, Japanese Patent Application Laid-Open No. 2005-32587 discloses a technology in which a relationship between the water content state of the cells and the open circuit voltage thereof is utilized. When the open circuit voltage of the cells is lower than a first threshold voltage, it is judged that the cells have a dry state. When the open circuit voltage is higher than a second threshold voltage, it is judged that the cells have an excessive water state, and the water content state of the cells is appropriately controlled (Patent Literature 5).
Here, especially when a cell voltage is excessively low, the cells degrade noticeably. Therefore, the minimum lower limit voltage of the cells, below which the cell voltage should not be, is set. If the cell voltage is below this minimum lower limit voltage, a predetermined amount of air is preferably supplied to raise the voltage (this processing will hereinafter be referred to as the “cell voltage lowering prevention processing”).    Patent Literature 1: Japanese Patent Application Laid-Open No. 2006-120375 (Paragraphs 0053 and 0054, etc.)    Patent Literature 2: Japanese Patent Application Laid-Open No. 2007-109569 (Paragraphs 0044 and 0045, etc.)    Patent Literature 3: Japanese Patent Application Laid-Open No. 2007-103023 (Paragraphs 0037 to 0044, etc.)    Patent Literature 4: Japanese Patent Application Laid-Open No. 2003-115318 (Paragraphs 0012 to 0014, etc.)    Patent Literature 5: Japanese Patent Application Laid-Open No. 2005-32587 (Paragraphs 0040 to 0058, etc.)